net/http: strip port from host in mux Handler

net/http: strip port from host in mux Handler

This change strips the port in mux.Handler before attempting to
match handlers and adds a test for a request with port.

CONNECT requests continue to use the original path and port.

Fixes #10463

Change-Id: Iff3a2ca2b7f1d884eca05a7262ad6b7dffbcc30f
Reviewed-on: https://go-review.googlesource.com/38194
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>